1. Technical Field
This invention relates to the art of making catalysts for treating automotive emissions, and more particularly to three-way catalysts that have high conversion efficiency for hydrocarbons, carbon monoxide, and nitrogen oxides.
2. Discussion of the Prior Art
Precious metals, including palladium, have been used as active catalyst materials in automotive emission devices. However, palladium, by itself, as the dominant or sole catalyst material, is limited in its effectiveness to promote the reduction of nitrogen oxide compounds at high temperatures and is not optimum in promoting oxidation of hydrocarbons at such temperatures. An example of such palladium usage is disclosed in U.S. Pat. No. 4,123,391. A variety of cost-increasing alternatives have been used to augment the capabilities of palladium, primarily by the use of other precious metals.
This invention has discovered that small amounts of titania (a nonprecious metal) uniquely applied will synergistically cooperate with lanthanum oxide in the presence of Pd to enhance the total catalytic effect for oxidation and reduction, particularly at very low temperatures and at very high temperatures. Titania has not been used heretofore as a catalyst enhancing ingredient nor have titania and lanthanum oxide been used together as catalyst enhancing ingredients. Titania has been incorporated in catalyst support materials for purposes of stabilizing such support materials, but prevented from entering into the catalytic process by coverings of other catalytically active ingredients (see U.S. Pat. Nos. 4,504,598 and 4,123,391). In some cases, titania has been layered on the support in large amounts prior to being covered with the catalytically active material (see U.S. Pat. No. 4,350,613).
Straight titania has been slurried onto prevalent precious metal materials (including palladium) in a continuous film (of about 0.003-0.01 inches) to make an oxidation catalyst more lead tolerant (see U.S. Pat. No. 4,650,782). Such catalyst construction was not intended for enhancement of reduction processes, the titania did not function as a reducing catalyst, and there was found no special catalytic synergism between titania and palladium.
Lanthanum oxide has been suggested for use as a substrate stabilizer as well as other rare earth oxides (see U.S. Pat. Nos. 4,624,941 and 4,283,308). In such disclosures, lanthanum oxide is dispersed as a sintered powder onto a substrate support such as alumina; the lanthanum oxide and support together are then covered with one or more noble metals. No improvement in catalytic activity was attributed to the presence of La.sub.2 O.sub.3. U.S. Pat. No. 4,791,091 found that by dispersing lanthanum oxide onto Al.sub.2 O.sub.3 in small particle size assisted the activity of rhodium in the temperature range of 200.degree.-460.degree. C., but failed to appreciate how lanthanum oxide could be used to increase catalytic activity of palladium at temperatures below 200.degree. C. and above 460.degree. C.